As semiconductor trends continue toward decreased size and increased packaging density, every aspect of semiconductor fabrication processes is scrutinized in an attempt to maximize efficiency in semiconductor fabrication and throughput. Many factors contribute to fabrication of a semiconductor. For example, at least one photolithographic process can be used during fabrication of a semiconductor. This particular factor in the fabrication process is highly scrutinized by the semiconductor industry in order to improve packaging density and precision in semiconductor structure.
Lithography is a process in semiconductor fabrication that generally relates to transfer of patterns between media. More specifically, lithography refers to transfer of patterns onto a thin film that has been deposited onto a substrate. The transferred patterns then act as a blueprint for desired circuit components. Typically, various patterns are transferred to a photoresist (e.g., radiation-sensitive film), which overlies the thin film on the substrate during an imaging process described as “exposure” of the photoresist layer. During exposure, the photoresist is subjected to an illumination source (e.g. UV-light, electron beam, X-ray), which passes through a pattern template, or reticle, to print the desired pattern in the photoresist. Upon exposure to the illumination source, radiation-sensitive qualities of the photoresist permit a chemical transformation in exposed areas of the photoresist, which in turn alters the solubility of the photoresist in exposed areas relative to that of unexposed areas. When a particular solvent developer is applied, exposed areas of the photoresist are dissolved and removed, resulting in a three-dimensional pattern in the photoresist layer. This pattern is at least a portion of the semiconductor device that contributes to final function and structure of the device, or wafer.
Techniques, equipment and monitoring systems have concentrated on preventing and/or decreasing defect occurrence within lithography processes. For example, aspects of resist processes that are typically monitored can comprise: whether the correct mask has been used; whether resist film qualities are acceptable (e.g., whether resist is free from contamination, scratches, bubbles, striations, . . .); whether image quality is adequate (e.g., good edge definition, line-width uniformity, and/or indications of bridging); whether critical dimensions are within specified tolerances; whether defect types and densities are recorded; and/or whether registration is within specified limits; etc. Such defect inspection task(s) have progressed into automated system(s) based on both automatic image processing and electrical signal processing.
Photoresist integrity must be maintained throughout the lithography process because any flaw or structural defect present on a patterned photoresist can be indelibly transferred to underlying layers during asubsequent etch process(es) wherein the photoresist is employed. One example of an undesirable structural defect is line-edge roughness (LER). LER refers to variations on sidewalls of features, which can result from variations of LER in the patterned photoresist. Many factors can contribute to LER in the photoresist, such as LER on chrome patterns residing on the reticle, image contrast in a system that generates the photoresist pattern, a plasma etch process that can be used to pattern the photoresist, inherent properties and/or weaknesses of the photoresist materials, and/or the photoresist processing method. Additionally, LER appearing in fabricated structures can result from damage to the patterned photoresist during an etch process. Furthermore, the smaller the wavelength employed to expose a photoresist, the greater the deleterious effects of LER.
Current methods of pattern line formation typically produce LER as an undesirable side effect. As lithographic techniques are pushed to their limits, smaller and smaller critical dimensions (CDs) are desired to maximize chip performance. Thus, chip manufacture is governed largely by wafer CD, which is defined as the smallest allowable width of, or space between, lines of circuitry in a semiconductor device. As methods of wafer manufacture are improved, wafer CD is decreased, which in turn requires finer and finer line edges to be produced. Line edges having a roughness that was acceptable just a year ago can detrimentally affect the performance of a chip exhibiting today's critical dimension standards.
Another example of undesirable structural defects are standing waves. Standing waves can occur when imperfections exist in a bottom anti-reflective layer (BARC). During an etch step performed, for example, on a resist layer that overlies a BARC layer, standing waves extant within the resist layer can be expressed. Such exposed standing waves can cause an etched surface (e.g., a sidewall) to display undulate topography, thus contributing to LER. Current methods of mitigating standing waves in a resist layer require the utilization of “perfect” BARCs, which are expensive and highly complicated with regard to implementation thereof during a photolithographic process(es).